The present invention relates generally to antennas for satellites and more particularly, to a reflector antenna system for a satellite which provides a plurality of antenna beams for full Earth field-of-view coverage from a geosynchronous orbit with each antenna beam having approximately equivalent beam characteristics and being substantially symmetrically shaped.
Communications satellites in geosynchronous orbit require high gain antennas for uplink and downlink communications with the Earth. A satellite uplink communications signal is transmitted to a satellite from one or more ground stations located on the Earth; and, a satellite downlink communications signal is transmitted from a satellite to one or more ground stations located on the Earth. The uplink and downlink signals are received and transmitted respectively at particular frequency bands which are typically in the ratio of about 3:2 (uplink frequency band: downlink frequency band) for Ka Band. The signals are also typically coded. A satellite is equipped with antennas or antenna systems to receive and transmit the uplink and downlink signals respectively. To minimize the number of satellites in a constellation and maximize communications capabilities, it is desirable for each satellite to have the capability to communicate with the locations on the Earth within the satellite""s field of view and to do so with high gain antenna beams.
FIG. 1 shows a simplified plan view of one antenna 10 used for high gain communications from satellites. This antenna 10 was detailed in the article Jorgensen, Rolf, et. al., xe2x80x9cA Dual Offset Reflector Multibeam Antenna for International Communications Satellite Applicationsxe2x80x9d, IEEE Transactions on Antennas and Propagation, Vol. AP-33, No. 12, December 1985. The antenna 10 is a dual offset cassegrain antenna having a main reflector 11, a subreflector 12 and a feed array 13. The feed array 13 consists of multiple feed horns with each feed horn generating an illumination beam 14 which is reflected from the subreflector 12 and main reflector 11 and directed toward a defined coverage cell on the Earth. The disadvantage with this antenna 10 is that it does not provide symmetrically shaped beams at wide scan angles.
The antenna 10 disclosed above has the additional disadvantage that it cannot provide high gain, adjacently located antenna beams. The above antenna 10 provides a single beam from each feed horn in the feed array 13. To provide high gain beams, the main reflector 11 must be efficiently illuminated. To do so requires large feed horns, with the location of each feed horn determining the location of a corresponding beam on the Earth. To provide beams which are adjacently located and completely cover the Earth""s field-of-view requires that all the feeds in the feed horn array 13 be physically positioned close together. If the feeds are not physically close together, the corresponding antenna beams will not be adjacently located and will be spaced too far apart on the Earth, with locations between antenna beams having no coverage. Large feed horns typically cannot be physically spaced close enough together within the antenna 10 to produce adjacent beams on the Earth. The above referenced antenna attempts to address this problem by using feed horns which are physically small so that the feed horns can be physically spaced close together. These smaller feed horns can produce adjacent beams but do not efficiently illuminate the reflectors 12, 11 resulting in high spillover losses and lower gain beams.
What is needed therefore is an efficient antenna system that provides a plurality of high gain, adjacent located antenna beams which cover the entire Earth field-of-view.
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides an antenna system for use on a spacecraft. In a first aspect, the antenna system comprises a feed array, a subreflector and a main reflector which are oriented to define a folded optics antenna geometry. In a second aspect the subreflector, main reflector and feed array are oriented to define a dual offset cassegrain antenna geometry where the feed array is offset from both the subreflector and the main reflector.
The feed array is comprised of a plurality of separate feeds which are aligned along a predetermined contour. Each feed is coupled to a feed network which acts to combine the illumination beams of a preselected number of feeds to produce a plurality of composite illumination beams each of which having a central ray. The central ray of each composite illumination beam is directed to be incident upon a separate preselected location on the subreflector. The subreflector is configured to receive each composite illumination beam at the preselected location and direct the central ray of each composite illumination beam towards the main reflector.
The main reflector is positioned to receive each composite illumination beam from the subreflector and direct each composite illumination beam in a preselected direction so that each composite illumination beam forms an antenna beam that impinges a predetermined coverage area on the Earth. Each antenna beam defines a separate coverage cell within a preselected coverage area.
In a third aspect, the position and orientation of the feeds, the subreflector and the main reflector provides adjacent antenna beams over a full Earth field of view coverage area where each antenna beam is approximately symmetrically shaped.
In a fourth aspect, the antenna system comprises a plurality of subreflector and main reflector combinations and a feed array associated with each subreflector and main reflector combination. Each subreflector and main reflector combination and associated feed array is oriented to define a separate folded optics antenna geometry.
In a fifth aspect, each subreflector and main reflector combination and associated feed array together are oriented to define a single dual offset cassegrain antenna wherein a plurality of dual offset cassegrain antenna comprise the antenna system.
Each feed array generates a plurality of illumination beams which form a plurality of associated antenna beams. The antenna beams from all the dual offset cassegrain antennas within the antenna system are interleaved. Each antenna beam defines a separate coverage cell in a coverage area with the coverage cells being arranged so that no coverage cell defined by an antenna beam associated with one dual offset cassegrain antenna is contiguous with another coverage cell defined by the same antenna beam associated with the same dual offset cassegrain antenna.